As a deaeration method of removing dissolved gas from a liquid, there is a method using a hollow fiber membrane. Hollow fiber membranes to be used in deaeration applications are required to exhibit solvent resistance and low dissolution properties to liquids to be treated, and polyolefins exhibiting these excellent properties are thus used as a membrane material in some cases.
For example, a composite membrane in which a homogeneous thin membrane formed by using an ethylene-higher olefin copolymer or a propylene-higher olefin copolymer, which is a low-density polyolefin, is laminated on a porous substrate is known (for example, see Patent Literatures 1 and 2). Patent Literature 3 discloses a hollow fiber membrane formed of poly(4-methylpentene-1).